The present invention relates generally to microelectronic assemblies having an integrated circuit die attached to a substrate by a plurality of solder connections, and more specifically relates to a microelectronic assembly having a bead of polymeric material disposed about the periphery of the integrated circuit die. The bead of polymeric material connects the die to the substrate and operates to reduce breakage of the die and electrical connections.
Many microelectronic assemblies are manufactured using the well-known flip chip on board (FCOB) technique. In FCOB assembly, an integrated circuit die is provided which includes a plurality of bond pads, with each of the bond pads having deposited thereon a solder bump. The die is turned over or flipped and is superimposed over a substrate having a plurality of bond pads, such that each of the solder bumps is aligned with a corresponding one of the bond pads on the substrate. The die and the substrate are then reflow-soldered together to form solder connections. The gap that remains between the downwardly facing die face and the upwardly facing substrate face is then filled using any one of a number of known underfill materials.
The underfill material, which typically contains silica or other particulates in a resin binder, serves to encapsulate the solder connections and serves to bond the die to the substrate. The underfill also increases the reliability of the microelectronic assembly during thermal cycling by enhancing the mechanical connection between the die and the substrate thereby mitigating the effects of thermal expansion problems.
Failure of integrated circuit dies from bending stresses, vibration stresses and stresses due to thermal expansion occurs most often at or near the die edges. Semiconductor dies are formed of an inflexible material that does not bend easily or resist multiple bending cycles. When mounted on a circuit board, a die will tend to break most often near the edge of the device when the board is flexed. The circuit board of hand-held devices may flex frequently from activation of buttons and rough handling during manufacturing and operation. Since electrical connections between the die and the board are often located near or at the edge of the die, the outermost located connections themselves are particularly vulnerable to failing when subjected to bending loads.
One prior art method of applying an underfill material has been to simply apply the underfill material to the assembly after the reflow soldering has been completed. The underfill material is drawn into the gap between the die and the substrate by capillary action. The assembly is then placed in an oven for curing. Typically, a small radius of underfill material, referred to as a fillet, adheres between the substrate and the lower edge portions of the die. An alternate example of using an underfill material includes first attaching a die to a substrate, tilting the assembly and allowing underfill material to flow under the die by the force of gravity. In both examples, since the epoxy becomes located for the most part underneath the body of the die, the peripheral edges or portions of the die are left relatively unsupported and vulnerable to stress.
A different method of bonding a semiconductor chip to a substrate uses a vacuum chamber to evacuate the gap between the chip and the substrate. A polymeric underfill material is forced into the gap when air is allowed to re-enter the chamber.
A technique of producing a die with high resistance to stress is obtained by providing a die within a package that also includes a base plate and cap. The die is adhesively attached to the base plate and a cap is affixed to the base plate such that an air gap is formed between the die and the cap. The base plate and die assembly is mounted to a substrate, such as a circuit board. The base plate tends to absorb stresses preferentially.
The methods described above do, to some extent, decrease the possibility of die and interconnect breakage due to stress by providing different means to support the die. However, these methods require additional manufacturing steps, materials and equipment. In some cases, encapsulating the die may itself exert stresses on the die, may be disadvantageous to marking techniques and may decrease the effective operating life of the die by reducing the capacity of the die to dissipate heat.
Accordingly, it would be desirable to provide a means of reducing die breakage that overcomes the disadvantages described above.